What is certificate of registration of citizen of the union?

I think that 'certificate of registration of citizen of the union' refers to residency certification. Usually you would obtain the padron first and are required to have this before you attend the Oficina de Extranjeros in Almeria City to obtain residency. As Garrucha comes under the Almeria City ambit I'm puzzled that you were rebuffed. Usually you can get a padron certificate from the local town hall if you can provide some proof of address - a tenancy agreement, a utility bill, etc. It may be worth you approaching a gestor for assistance.

I second that.  They are referring to Residencia, which you cannot obtain without a Padrón ... someone st the Ayuntamiento is either new, or didn’t want to do some work.  There is no requirement to have Residencia prior to getting your Padrón, only proof of your living in the area you are registering, normally deeds to a house or a tenancy agreement is sufficient for this, but each Ayuntamiento has their own rules.  Yes - if Ayuntamiento insist on Residencia, then get a Gestor to help you as you CANNOT get Residencia without first registering for a Padrón (Certificado de Empadronamiento)

Saludos, Darío